Compression evaporator control and method therefor



Nov; 11, 1958 H. 6. EHRISMAN 2,859,309

COMPRESSION EVAPORATOR CONTROL AND METHOD THEREFOR Filed Feb. 2, 1955INVENTOR Hen/7 0. E/zrvsmam ATTO GONDENSATE United States Patent ICOMPRESSION EVAPURATOR CONTROL AND METHOD THEREFOR Application February2, 1955, Serial No. 485,697

' 5 Claims. (Cl. 159--44) This invention relates to compressionevaporation and more particularly to a novel method and apparatus forcontrolling the addition of auxiliary heat to such an evaporator. Sincemost evaporation processes are concerned with the evaporation of aqueoussolutions the present invention will be described in connection with itsuse for evaporating such .a solution, although as the descriptionproceeds it will be evident that the invention can also be used withadvantage in connection with the evaporation of non-aqueous liquors aswell.

In recent years there has been an increasing interest in compressionevaporation as .a method of increasing the thermal efiiciency of variousevaporation processes. In accordance with this method vapors distilledfrom the liquid being evaporated are removed from the evaporator body,compressed by a suitable compressor, and returned to the evaporatorsteam chest to be used as a heating medium for evaporating furtherquantities of the liquor to be evaporated. In such a compressionevaporation process it is' customary to preheat the feed liquor withcondensate from the evaporator steam chest, and if this heat exchangebetween feed liquor and condensate is carried out efficiently, anevaporative process is achieved which has exceptionally high thermalefficiency, i. e. a

process which will vaporize a relatively large quantity of water perunit of heat input.

It is evident however that even with the high efficiencies obtainablewith such a compression evaporator some heat losses inevitably occur,and accordingly it is necessary that auxiliary or makeup heat be addedin order to carry out the evaporation on a continuous basis. It isaccordingly an object of the present invention to provide an improvedmethod and apparatus for supplying auxiliary heat to a compressionevaporator. It is another object of the invention to control the supplyof heat to a compression evaporator in such manner that the evaporationproceeds in a more nearly uniform manner than has heretof-ore beenachieved. It is still another object of the invention to provide amethod and apparatus for controlling the supply of heat to the steamchest of a compression evaporator in such manner as to maintain thepressure in the vapor space of the evaporator substantially constant.Other objects of the invention will be in part obvious and in partpointed out hereafter.

The many objects and advantages of the present invention can best beunderstood and appreciated by reference to the accompanying drawingwhich illustrates diagrammatically apparatus incorporating a preferredembodiment of the present invention and capable of carrying out themethod of the invention.

Referring to the drawing, the numeral generally designates an evaporatorbody having a heat exchange section 12 and vapor body 14. The evaporatorbody 10 is a conventional vertical tube, natural convection evaporatorwherein the heat exchange section 12 contains a bundle of vertical tubes16. Although only two tubes 16 are shown for purposes of illustration,it will be understood that the tube bundle comprises a large number oftubes largely filling the space within the heat exchange section 12. Thespace around tubes 16 is supplied with steam through a pipe 18 and willbe referred to herein for convenience as the steam chest 20 of theevaporator.

The liquor to be evaporated is supplied to the base of the heat exchangesection 12 through a pipe 22 and flows up into the tubes 16. Within thetubes it is heated by a steam in the steam chest 20 and partiallyvaporized. As a result of this vaporization process a mixture of vaporand hot liquor is delivered from the upper ends of the tubes 16 andimpinges against a baflie 24 in the vapor body 14. The liquor and vaporare separated in the vapor body 14 and the liquor flowsdownwardlythrough a pipe 26 and is recycled through feed pipe 22 to thebottom of heat exchange section 12. Concentrated liquor is continuouslywithdrawn from vapor body 14 through a pipe 6% and pumped by .a pump 62through a product pipe 64 to a suitable point of storage or use.

As indicated above, the present evaporator is a compression evaporatorwherein the vapor is compressed and used as a heating medium in thesteam chest of the evapo rator. Still referring to the drawing, thevapor leaves the vapor body 14 through a pipe 28 and flows to the inletof a compressor 30 that is driven by a steam turbine 32. The compressedvapor is delivered by the compressor 30 to the pipe 18 through which itflows to the steam chest 20 of the evaporator body.

In order to improve the thermal efliciency of the unit the feed liquoris preheated by being caused to flow in heat exchange relationship withthe condensate formed in the steam chest 20. More particularly,condensate is withdrawn from steam chest 20 near the bottom thereofthrough a pipe 34 and flows successively to the shell sides of the heatexchangers 36 and 38 and thence to a suitable point of disposal throughthe pipe 40. Feed liquor to be evaporated enters the :system through apipe 42 and flows through the tube sides of heat exchangers 38 and 36 insequence to the feed pipe 22 referred to above. It is thus apparent thatthe feed liquor and condensate are caused to flow in generallycountercurrent heat exchange relation to cause the liquor to be heatedby the sensible heat in the condensate leaving the system.

The rate of introduction of feed liquor into the evaporator body isdetermined by the liquor level in the heat exchange portion 12 of theevaporator body 10 and control mechanism is provided for maintainingthis liquor level at the desired point. Thus a gauge pipe 44 is providedwhich is connected at its upper end to vapor body 14 and at its lowerend to the lower portion of heat exchange section 12 below the bottom ofthe tube bundle 16. Mounted in the lower portion of gauge pipe 44 thereis a differential pressure responsive device 46 which may be ofconventional construction, and for example of the specific constructionshown in Cook Patent 2,539,892. Thus if a device such as that shown inthe Cook patent is used the passage 111 of the Cook patent is connectedto the vertical portion of the pipe 44 and the passage 98 of the Cookpatent is connected to the liquid space at the bottom of evaporator 10.

The diiferential pressure responsive device 46 is eflectively responsiveto the height of the liquor level in the tubes of tube bundle 16 andoperates to generate a pneumatic pressure proportional to the liquorlevel. This pneumatic pressure is transmitted by a pipe 48 to acontroller 50 which is in turn connected by a pipe 52 to a pneumaticallyoperated regulating valve 54 located in the feed liquor pipe 22. (If adevice like the Cook device is used, pipe 30 of the Cook patentcorresponds with pipe 48 of the present application.) The controller 50operates to generate a pneumatic pressure which is a function of theliquor level in the tubes 16 and supplies this pneumatic pressure to thevalve 54 to so regulate the feed liquor rate as to maintain the desiredliquor level. in. the evaporator. Pneumatically operated controllerscapable of'operating. inthisway are conventional and well-knownin theart andhence the construction of controller 50 need not be described indetail herein.

In like manner the rate of withdrawal of condensate from steam chest 20is regulated so as to maintain the steam chest substantially free fromaccumulated condensate. More particularly the pipe 34 has a U-section 35to the right-hand leg. of which a differential pressure responsivedevice 56 is connected by pipes 57 and 59. The device 56 may be similarto the device 46 and generates. a pneumatic pressure that is suppliedthrough pipe 61 to a regulating valve 58, also located in the pipe 3 3.(If a device such as that shown in Cook Patent 2,539,892 is used thepassage 111 of the Cook patent is connected to pipe 57 and the passage.98 of the Cook patent is connected-to pipe h.) The differential pressureresponsive device 56 operates to supply to valve 58 a pneumatic pressurewhich positions the regulating valve 58 to maintain the flow of.condensate through the pipe 34 at such a value that the condensate levelin the right-hand leg of U-section 35. remains between the points atwhich pipes 57 and 59 are connected to pipe 34. Thus condensate formedin steam chest 20 immediately flows into pipe 34 and no substantialamount of condensate accumulates in the steam chest.

As pointed out above, in a compression evaporation system of the typeshown certain heat losses occur and hence auxiliary heat must besupplied to permit the evaporator to operate on a continuous basis. Thisauxiliary heat can manifestly be obtained from any of various sources.However in the preferred embodiment of the invention here beingdescribed the compressor 30 is driven by a steam turbine 32 and theexhaust steam from the steam turbine is used as a source of thisauxiliary heat. More particularly the steam turbine 32 is driven by highpressure steam supplied through a pipe 66 which contains a regulatingvalve 68. Exhaust steam from turbine 32 flows through a pipe 70 to a lowpressure steam system (not shown) which is maintained at a pressure ofsay 50 lbs. per square inch. A portion of the exhaust steam flowsthrough a branch pipe 72 containing a regulating valve 74 to the steamsupply pipe 18 that supplies steam to the steam chest 20 of theevaporator. The flow of auxiliary steam from the exhaust of steamturbine 32 to steam chest 20 is controlled by a controller 76 in amanner that is further described hereafter.

As an incident of the normal operation of evaporators, the steam supplyto the evaporator commonly contains small quantities of fixed gases andthese gases accumulate over a period of time in the evaporator steamchest. To

effect removal of thesefixed gases the present evaporator system isprovided with a vent system comprising a pipe 78 connected to steamchest 20 just above the condensate level. The pipe 78 contains apneumatically operated regulating valve 80 and is provided with aby-pass 82 around the valve 80 which contains an orifice 84. The orifice84 is so selected as to withdraw the vent gas and admixed steam fromsteam chest 20 at such a rate as to maintain the vent gas concentrationwithin the steam chest at a relatively low value with the valve $0closed. The valve 80 forms part of the control system for the evaporatorwhich will now be described.

In the operation of an evaporator it is important in securing efiicientevaporation that the evaporation rate be maintained relatively uniformand that vaporization occur smoothly and continuously. In accordancewith the present invention such uniform continuous operation is obtainedby maintaining the pressure in. the vapbispace of vapor body 14substantially constant, and this constant pressure in the vapor spacewithin body 14 is achieved by conjoint regulation of the steam supplythrough pipe 72 and of steam withdrawn from steam chest 20 through pipe78. More particularly the gauge pipe 44 is provided near its upper endwith a pressure responsive device 38 which may be like the devices 46and 55 previously referred to but which is exposed to atmosphere on oneside thereof and hence measures the pressure in the vapor space 14. (Ifa device such as that shown in Cook Patent 2,539,892 is used the passage111 of the Cook patent is connected to the upper end of gauge pipe 44and the passage 98 of the Cook patent is left open to the atmosphere.)The device 88 generates a pneumatic pressure proportional to thepressure in vapor space 14 which pneumatic pressure is supplied as acontrol pressure to the controller 76. The controller 76 in response tothe pressure values received from device 88 generates a pneumaticpressure that is applied through the pipe 9i) to the valve 74 andthrough the pipe 92 to the valve 8t). The regulating valves 74 and areso adjusted in relation to the pressure supplied by the controller 76that the valve 74 is open only when the controlled pressure, i. e. thepressure within the vapor body 14, is below the desired value and thevalve 80 is open only when the controlled pressure is above the desiredvalue.

' The cooperative action of the valves 74 and 80 and controller 76 canbest be clarified by a specific example.

Assume for example that:

(l) The desired pressure in the vapor space of the evaporator is 5 lbs.per square inch.

(2) When the pressure in vapor body 14 is stabilized at 5 lbs. persquare inch the controller 76 generates a pneumatic pressure of 9 lbs.per square inch.

(3) As the pressure in vapor body 14 varies the output pressurecontroller 76 can vary between 3 lbs. per square inch and 15 lbs. persquare inch.

(4) As the vapor body pressure increases the output pressure ofcontroller 76 decreases and vice versa.

(5) A change of 1 lb. per square inch in vapor body pressure correspondsto a change of 2 lbs. per square inch in controller output pressure.

(6) At the time being considered the vapor pressure in the vapor body is2 lbs. per square inch.

It is evident that under the conditions set forth above the controller76 will be supplying its maximum air pressure of 15 lbs. per square inchto both valves 74 and 80. The valve 74 is an air-to-open valve and is soadjusted that it is wide open with 15 lbs. air pressure on itsdiaphragm. The valve 80 is an air-toclose valve and is adjusted so thatit is fully closed with 15 lbs. air pressure on its diaphragm and doesnot begin to open until the pressure on its diaphragm drops to 9 lbs.per square inch.

With the valve 74 Wide open a full flow of auxiliary steam will occurthrough pipe 72 and thence through pipe 18 to steam chest 20 thusincreasing the rate of evaporation and the pressure within the vaporspace of the vapor body 14. This increase in vapor pressure will bereflected in a decrease in the output pneumatic pressure of controller76 and the valve 74 will be gradually closed. When the vapor pressure inthe evaporator reaches 5 lbs. per square inch both valves 74 and 80 willbe fully closed. If thereafter the vapor pressure of the evaporatorrises above its control value, that is 5 lbs. per square inch,controller 76 will generate a pressure below 9 lbs. per square inch andvalve 80 will begin to open. As a result of the opening of the valve 81steam will be vented from steam chest 20 thus decreasing the rate ofevaporation of the liquor in the evaporator and tending to reduce thevapor pressure therein. t will thus be evident that the controller 76operates to regulate the valves 74 and 80 conjointly to maintain thepressure in the evaporator vapor space substantially at a desired value.

From the foregoing description it should be apparent that the presentinvention provides apparatus capable of achieving the objectives setforth at the beginning of the present specification. It is of course tobe understood that the embodiment of the invention described herein isillustrative only and that numerous changes can be made therein withoutdeparting from the spirit of the invention as set forth in the appendedclaims.

I claim:

1. In a compression evaporator of the type wherein water vapor iswithdrawn from the vapor space of the evaporator, compressed by acompressor, and returned to the evaporator steam chest, apparatus formaintaining the pressure 'in said vapor space substantially constant,comprising, in combination, an auxiliary steam conduit connected tosupply steam to said steam chest, a first regulating valve in saidauxiliary steam conduit, a vent conduit connected to said steam chestfor venting steam therefrom, a second regulating valve in said ventconduit, and control means responsive to the pressure in said vaporspace and connected to. operate said first and second regulating valves,said control means operating said first regulating .yalve to close saidvalve with increasing'pressure in said vapor space and vice versa andoperating said second regulating valve to open said valve withincreasing pressure in said vapor space and vice versa, said controlmeans conjointly operating said two valves to maintain the pressure insaid vapor space substantially constant.

' 2. Apparatus according to claim 1 and wherein said control means uponan increase in pressure in said vapor space causes said auxiliary steamvalve to close completely before permitting said vent valve to open andupon a decrease in pressure in said vapor space causes said vent valveto close completely before permitting said auxiliary steam valve toopen.

3. Apparatus according to claim 1 and wherein said compressor is drivenby a steam engine and said auxiliary steam conduit is connected to theexhaust of said steam engine, whereby exhaust steam from said engine isused in the auxiliary steam for said evaporator.

4. The method of operating a compression evaporator of the type whereinwater vapor is'withdrawn from the vapor space of the evaporator,compressed by a steam engine-driven compressor, and returned to theevaporator steam chest to maintain the pressure in said vapor spacesubstantially at a predetermined value, which comprises the steps of,supplying exhaust steam from the exhaust of said steam engine to saidevaporator steam chest when the vapor space pressure is below saidpredetermined value and venting steam from said steam chest when thevapor space pressure is above said predetermined value.

5. The method of operating a compression evaporator of the type whereinwater vapor is drawn from the vapor space of the evaporator, compressedby a compressor, and returned to the evaporator steam chest to maintainthe pressure in said vapor space at substantially a predetermined,desired value, which comprises the steps of supplying auxiliary steam tosaid steam chest, venting steam from the steam chest of said evaporatorand conjointly regulating in response to variations of the pressure inthe vapor space of said evaporator the flow of auxiliary steam and theflow of vent steam to increase the auxiliary steam flow when the vaporspace pressure is below said desired value and falling, and to increasethe vent steam flow when the vapor space pressure is above said desiredvalue and rising to maintain the vapor space pressure substantially atsaid desired value.

References Cited in the file of this patent UNITED STATES PATENTS

